The invention is directed to an analog-to-digital converter for consecutively converting analog signals received in parallel on a plurality of incoming lines into corresponding binary encoded digital output signals.
More specifically, the invention is related to a converter as above for use with a pulse coded time division multiplex telephone exchange system. In such a system the incoming lines comprise speech channels carrying analog voice signals generated by electro-acoustical or electro-optical converters. The continuous analog voice signals have to be sampled for band-pass filtering and encoding. From the analog voice signal a binary coded digital signal is derived which is transmitted across transmission lines to a corresponding receiving circuit arrangement where the original signal is reconstructed from the transmitted signal.
Filtering of a voice signal can be established in a conventional manner in using an integrated filter circuit limiting the band width of the voice signals to a predetermined frequency band identical with the band width of a voice channel. The filter circuit also comprises an output amplifier having an output coupled to an input of a selector unit controlling the sampling operation. This selector unit is associated with a whole group of such speech channels limited by the number of signal inputs. At an output of the selector unit a sampling probe of a voice signal is delivered for digital encoding.
For encoding the sampled signal a known converting method, the so-called iterative method can be employed. The output of the selector unit is connected to a grounded storage capacitor and in parallel to a first input of a comparator having a second input for receiving a current reference signal. This reference signal, in the given case, is generated in such a manner that at a starting time a digital figure is generated which is converted into its corresponding analog voltage. This internally generated analog voltage is supplied to the comparator as reference voltage thus being compared with the current sampling signal. In accordance with the condition of the comparator by means of its output signal, the previously generated digital number is adjusted to be higher or smaller and is converted into a new corresponding analog reference signal again applied to the comparator. This scheme of iteratively adjusting a presently established digital number step by step is continued until the approximation of this digital value to the stored analog value is sufficiently exact. The final digital number then represents the converted digital value of the sampling probe which is presently buffered in the storage capacitor.
Obviously, the voltage drop across the storage capacitor representing the analog value of a sampling probe may not change because of a leakage loss as long as the converting operation continues. Such a change also could be effected by the off-set failure of the mentioned output amplifier. For eliminating the influence of such an off-set failure in each input circuit for sampling and storing an analog value a rejector circuit has to be provided. Additionally, it is desirable that interfering voltages having a frequency below the speech channel band width, as for example noise induced by power mains, are attenuated.
If sampling probes of several output amplifiers each associated with a different speech channel are consecutively stored without a timely overlapping by one and the same storage capacitor cross-talk attenuation in such a system for digital transmission of voice can be reduced as much that the cross-talk level can interfere with the transmission of the useful signals. Furthermore, if integrated selector units are provided for the sampling operation, the switch-on resistance of such selector units has to be as high as to avoid overdriving the output stage of the output amplifier while setting up a through-connection. In such a situation, a time constant determined by the switch-on resistance and the necessary capacity of the storage capacitor can assume such a value that the storage capacitor cannot be discharged sufficiently between sampling intervals. The residual charge of the storage capacitor then results in a cross-talk effect.
It is, therefore, an object of the present invention to provide an improved analog-to-digital converter designed in such a manner that high cross-talk attenuation is obtained by minimal outlay of circuit design.
Another object of the present invention is to provide a converter as above of a design which is suitable for use with integrated circuit technology.